1. Field of the Invention
The present invention relates to a cleaning process for a semiconductor manufacturing system, and particularly to removing contaminants from a semiconductor substrate surface during processing. More particularly, the invention relates to removal of contaminants from a semiconductor wafer by in-situ generation of oxidizing species. These active species are generated by the simultaneous application of ultra-violet radiation and chemicals containing oxidants such as hydrogen peroxide and dissolved ozone. The invention also relates to enhancement of the oxidation reactions from removing contaminants by the application of ultrasonic or megasonic agitation. Further, the invention relates to the removal of reaction products by rotation of the substrate.
2. Description of Related Art
The fabrication of Integrated Circuits (IC's) on a semiconductor substrate can properly be divided into two sets of processes. The first set of these processes, called in industry parlance, “Front End-Of-Line” (FEOL), is used to lay the building blocks of the transistors that make up an IC. In this set of processes, various selected areas of the substrate are exposed to implantation of impurities such as phosphorous, boron, or arsenic in order to create p-type or n-type regions. In the manufacturing cycle for an IC, a sequence of discrete steps are used to define these regions so that only selected areas of the semiconductor are subjected to this process of doping. This begins with the deposition of a layer of photoresist. The photoresist layer is dried and cured after being deposited. The photoresist is photoactive, and can be modified by exposure to selected forms of radiant energy. Exposure of the resist is performed in a photolithography step where the substrate is exposed to radiant energy of selected wavelengths through a mask. This mask defines those areas of the photoresist-coated substrate, which are subjected to the radiation, and those that are not. Typically the areas of photoresist that are subject to the radiation are modified and can be removed by developing in much the same way as in photography. This method of pattern transfer (from mask to substrate) leaves photoresist covering those areas of the substrate that were shielded by the mask.
Ion implantation is then used to drive dopants such as phosphorous, boron or arsenic into those areas of the substrate that are not protected by the photoresist. Subsequent to this step, all the photoresist must be removed before the substrates are annealed, oxidized or processed in diffusion furnaces. Currently, post-implant photoresist removal is performed in one of two ways. Wet processing involves the use of a mixture of sulfuric acid and hydrogen peroxide to remove the resist and other organic contaminants that might be present. Photoresist can also be removed using dry processes, typically involving the use of plasma oxidation. In a plasma process, a source of oxidant, such as oxygen, is introduced into a plasma field excited either by radio frequency or microwave frequency energy. The semiconductor substrate is also heated to temperatures in excess of 200° C. to enhance the rate of the photoresist removal. In the example cited above, diatomic oxygen breaks up into intermediate mono-atomic oxygen radicals, which then act on the organic photoresist. In most cases, the substrate may still be dipped in a mixture of sulfuric acid and hydrogen peroxide after a plasma process is used to remove photoresist. This sequence of steps may be repeated more than once depending on the complexity of the integrated circuit that is being developed by repeated patterning of regions for introduction of various dopants.
The recent process trends in the manufacture of IC's have caused an increase in the level of doping. This has been achieved by increasing the energy and density of the ion flux directed at the substrate during the implantation process. As a consequence, the surface of the photoresist that shields certain areas of the substrate from the ion implantation process is itself modified. Due to the high energy and flux density, surface layers of the photoresist undergo chemical and physical modification. The higher temperatures resulting from the ion bombardment cause further baking and hardening of the top layer. Also, the ion flux causes implantation of the resist with the dopant atoms. Moreover, the photoresist undergoes significant cross-linking and becomes more resistant to post-implant removal processes.
Dry photoresist stripping by using plasma processes has had significant difficulty in overcoming these crusted surface layers. One problem frequently noted in dry stripping is that of crust “popping.” This occurs when the substrate is heated for dry stripping, and the solvent in the soft photoresist bulk underlying the crust begins to evaporate. The solvent vapors are released by breaking through the brittle crust, which in turn causes a significant distribution of particulate contamination on the substrate. Plasma processes have had to be modified to include a degree of Reactive Ion Etching (RIE) so that the crust may be removed at low temperature before the softer bulk resist is striped using a traditional downstream plasma process. RIE, involving the acceleration of oxygen or argon ions towards the substrate, which is held under an electrical bias, has been known to cause charge damage to the sensitive devices created on the substrate.
After the photoresist has been removed, either by wet processing in a sulfuric-peroxide mixture or by dry plasma stripping, the substrate is subjected to an RCA clean. This is actually a sequence of wet processes aimed at removing various kinds of contaminants prior to a high temperature process such as diffusion, oxidation or annealing. The RCA typically incorporates the following elements: a) preliminary cleaning in a mixture of sulfuric acid and hydrogen peroxide to remove organic contaminants, including photoresist if so desired; b) removal of organic contaminants, particulates, and certain metals by immersion in an aqueous solution of ammonium hydroxide and hydrogen peroxide; c) removal of thin oxide film generated by the previous step by immersion in an aqueous hydrofluoric acid (unless layer is already coated with oxide); d) removal of the remaining metallic contaminants by immersion in an aqueous solution of hydrochloric acid and hydrogen peroxide; and e) drying.
Following each of steps a through d, the substrates are immersed in de-ionized water so that they may be rinsed of residual chemicals. Spin-spray processes, which use far smaller quantities of chemicals, may replace the immersion processes.
Recent process trends have been aimed at replacing the sulphuric acid/hydrogen peroxide mixture with ozonated de-ionized water. Another process trend has been to reduce the concentration of the ammonium hydroxide/hydrogen peroxide mixture and the hydrochloric acid/hydrogen peroxide mixture solutions that comprise the RCA clean by employing megasonic and ultrasonic enhancement techniques.
Megasonic cleaning technology was developed to complement the RCA wet chemical process and to enhance removal of organic films and particles. In the megasonic process, wafers are immersed in water, alcohols, or dilute SC1 solutions and exposed to sonic waves, typically in the range of 850 to 900 KHz. Ultrasonic cleaning works on the same principle, but uses liquid cavitation frequencies in the range of 20 to 80 KHz and power densities fifty-times greater than that used for megasonic applications. Both ultrasonic and megasonic cleaning processes operate on the principle of introducing cavitation and miro-acoustic streaming to enhance rates of mass transfer. All wet processes are handicapped by the slow rates of diffusion of reactants to the substrate surface and the diffusion of reaction products away from the cleaned surface. These diffusion rates are slow because of the thick hydrodynamic boundary layers that are typically established over the substrate surface as the cleaning solution flows over them. To overcome this, megasonic and ultrasonic transducers create additional mechanical agitation. The sound energy that is coupled into the liquids by these transducers cause cavitation, i.e., generation of gas and vapor bubbles in the boundary layer. The subsequent collapse of these bubbles generates streaming effects within the relatively quiescent hydrodynamic boundary layer, thereby causing more agitation. Megasonic excitation adds the effect known as micro-acoustic streaming, whereby pulsating flow caused by the sound waves are transferred into the hydrodynamic boundary layer with a similar effect.
A traditional method for treating semiconductor substrates and removing residual organic films includes the use of ozonated water or other ozonated reagents. This method is typically comprised of the following distinct process steps: a) generation of ozone; b) dissolution of ozone in de-ionized water or reagent; and c) introduction of ozonated water or reagent into the reactor or chamber. However, each of the above steps is neither effective nor efficient. Ozone is generally created by corona discharge in oxygen. This ozone generation process remains extremely inefficient. At best, only a few percent of all oxygen is converted to ozone. Moreover, the ozone generated has a short half-life, and has to be dissolved into de-ionized water and delivered to the process chamber very quickly. Ozone is dissolved in de-ionized water or other reagent through a contactor, such as a bubbler or membrane module. Ozone dissolution is a slow process and the de-ionized water may not reach saturation in the contractor. At best, commercial ozone generators achieve concentrations on the order of forty (40) parts per million in de-ionized water. This represents a very low rate of ozone generation, typically up to 5%, and a low level of saturation in the ozone generator.
Transfer of the ozonated water from the contractor into the substrate-processing chamber can cause further loss of ozone by dissociation to oxygen. Furthermore, transport of these small concentrations of dissolved ozone to the reaction sites on the semiconductor substrate is subject to mass transport restrictions. Although various methods such as ultrasonic and megasonic excitation have been introduced to enhance the mass transport, none of the current methods enhance the availability of ozone near the reaction sites. Thus, because of this small amount of ozone in the solution, there can only be a limited amount of oxidizing species.
Another recent trend has seen the increased use of ozonated de-ionized water to replace hydrogen peroxide in the cleaning process. Some processes use ozone dissolved in de-ionized water and mixed with either hydrochloric or hydrofluoric acid to develop a uniform and contaminant-free silicon dioxide layer at the surface of the semiconductor substrate.
In addition to the FEOL processes described above, there are a number of Back-End-Of-Line (BEOL) processes that require stripping of photoresist and removing of organic residues that result from etch processes. The Back End-of-Line (BEOL) refers to that set of process steps which connects the various transistors that were defined in the Front End-of-Line. To provide for this interconnection, alternating layers of dielectric material and metal are deposited and etched to form microscopic “wiring” for the integrated circuit. In a typical dual-Damascene integration, a layer of dielectric material is deposited on the substrate. This dielectric material is then patterned using photoresist deposition, curing, photolithography, and development in much the same way as in the Front End-of-Line. However, instead of ion implementation, a plasma process is used to etch features into dielectric material. In one example of a dual-Damascene integration scheme, vias are first etched into the dielectric. These vias are the holes used to establish conductive contact between layers of metal on either side of the dielectric layer. After vias have been etched, the photoresist must be removed and the substrate cleaned prior to the next patterning and etching process, e.g., patterning and etching for trenches. Trenches constitute the next layer of metal. After trenches have been etched, the photoresist used to delineate areas for etching also has to be removed. Once trenches and vias have been etched into the dielectric, conductive barriers and metal layers are deposited first to make contact with the metal layer underlying the dielectric, and then to build up the next metal layer. By repeating this sequence of deposition and etching up to 10 layers of metal are usually created, thus providing the wiring for the integrated circuit.
In the dual-Damascene scheme described above, the photoresist layers are also subject to heavy bombardment of ions during etching, The dielectric materials are typically etched using a fluorine containing etchant gas. The resulting plasma contains many ionic fluorine species that chemically and physically alter the photoresist. High-density plasma is finding greater application in etching and typically results in a larger flux of ions at the substrate surface. This causes cross-linking and fluorination of the photoresist layer at the top surface. In a manner similar to that seen in the Front End-of-Line, a crust of cross-linked polymer is formed on top of softer unmodified photoresist. This crust is very stable and impervious to traditional downstream oxygen plasmas that have been used for photoresist removal.
Typically, these resist layers are removed using RIE plasma processes with oxygen or fluorine-containing species as the reactants. Reducing chemistries employing hydrogen, forming gas (a mixture of N2 and H2), and forming ammonia have also been employed for stripping resist and removing post-etch residue. Plasma processes that have been commonly used for traditional structures that employ SiO2 as the inter-layer dielectric do not result in any damage to the dielectric material. However, the introduction of carbon-doped oxides, such as CORAL® from Novellus Systems, Inc., BLACK DIAMOND® from Applied Materials, and the like, has resulted in associated problems with plasma-based strip and clean processes. The aggressive atomic and ionic species generated by plasmas have modified the structure of these inter-layer dielectric materials, and consequently augment the value of critical material properties, such as their associated dielectric constant (κ).
As device geometries shrink, the semiconductor roadmap calls for the introduction of materials with low dielectric constant values. Some organic dielectric materials are now currently being tested for future semiconductor devices. These organic materials, e.g., SiLK® from Dow Chemicals, are similar in their chemical composition and reactivity to the photoresist layers used to define etching patterns. Use of plasma processes, therefore, may not be possible at all for removing photoresist and post-etch residue from substrates incorporating organic dielectric materials. The use of oxygen plasma, for example, has been ruled out by the similar etch rates demonstrated by SiLK® and photoresist in these conditions. Consequently, a need exists in the art for the development of alternative stripping and cleaning processes that do not affect or materially alter the properties of the dielectric materials.
Bearing in mind the problems and deficiencies of the prior art for both FEOL and BEOL photoresist strip processes, it is an objective of the present invention to provide a method for removal of photoresist and organic contamination without damaging the underlying layers. In the FEOL processes, it is the objective of this process to remove heavily cross-linked and implanted resist crust without causing particulate contamination by “popping.” It is also the objective of this invention to provide for the photoresist removal without subjecting the substrate to the deleterious effects of reactive ion etching.
It is a further object of the present invention to integrate the photoresist removal step with the subsequent cleaning steps employed in the RCA clean process.
Another object of the present invention is to successfully integrate the entire photoresist strip and RCA clean process currently practiced in one chamber. It is a further object of the present invention to allow for a reactor equipped with sufficient functionality such that all the various cleaning steps may be performed in sequence in one chamber without the need for handling between each of the steps that constitute an RCA clean.
Yet another object of the present invention is to enhance the rate of resist removal by providing for ultrasonic or megasonic excitation near the substrate, and preferably coupled to the substrate through a thin layer of reactant fluid on its surface. It is an objective of this invention to enhance transport of active ingredients from the bulk reactant fluid to the surface of the substrate by means of megasonic and ultrasonic excitation. It is also the corresponding objective of this invention to enhance transport of the products of reaction away from the substrate surface by combination of megasonic and ultrasonic excitation with centrifugal forces resulting from substrate rotation. It is another object of the present invention to provide for the generation of active oxidizing species near the substrate surface so that they may be available in close proximity to the reaction sites. It is the further object of this invention to increase the source materials for these active species by employing reagents that are highly soluble in aqueous media.
In the BEOL processes, it is an object of the present invention to remove the heavily cross-lined and fluorinated resist crust without causing damage to the underlying dielectric layers. It is a further object of the present invention to remove post-etch residue from the vias and trenches that have been etched in a prior step.
Yet another object of the present invention is to integrate in one chamber all the cleaning and drying steps necessary for the removal of photoresist and organic residues from vias and trenches. It is a further objective of the invention to provide for removal of residue from within vias and trenches by providing for ultrasonic and megasonic excitation. Another object of the present invention is to remove resist and residue by providing for the in-situ generation of oxidizing species from water-soluble reagents.